This invention relates to a technique for restoring traffic in a SONET/SDH ring network.
Most major providers of telecommunications services, such as ATandT, now rely primarily on optical fiber cables to carry long-haul and even some short haul traffic. As compared to copper cable and microwave transmission facilities previously used to carry telecommunication traffic, optical fiber cables offer higher traffic capacity and provide higher quality. The development of large networks of fiber optic cables has prompted the need for a fast and efficient mechanism for accomplishing restoration in case of a failure, due to a severed fiber optic cable or an inoperative piece of terminal equipment. In the early 1990""s, telecommunication network service providers used mesh-based restoration architectures. U.S. Pat. No. 5,182,744, issued on Jun. 26,1993 in the name of James Askew et al. and assigned to ATandT, (incorporated by reference herein) describes and claims a mesh-based restoration architecture, known commercially as xe2x80x9cFASTAR.xe2x80x9d The FASTAR system has been deployed with much success by ATandT and remains in use today.
In the mid-1990""s, many telecommunications network service providers, including ATandT, began large-scale deployment of SONET/SDH fiber optic ring networks in response to an increase in data communications traffic. A typical SONET/SDH fiber optic ring network includes a 1X1 configuration with a protection ring associated with each service ring. A Lightwave Capacity Terminal monitors traffic on each service ring, and in case of a failure, the terminal switches traffic onto a protection ring, usually within 60-200 milliseconds. Such fast switching is critical for data communications since data is more susceptible to delays as long delays may cause synchronization failures.
In practice, SONET/SDH rings may not always offer the high reliability that they claim. For example, the protection rings may not always be available because of the nature of the Lightwave Capacity Terminals. For example, the protection ring may not be effective to carry traffic if the protection switching occurs simultaneously with a cable cut. Another factor that limits the reliability of SONET/SDH rings is the increasing size of such rings due to right-of-way restrictions. The increased size of such rings has increased the occurrence of double optical fiber cuts that affect both the service and protection rings.
Thus, there is a need for technique that affords increased reliability in an SONET/SDH ring.
Briefly, the present invention provides a method for restoring traffic in a network that includes (i) at least one service ring configured of first links that each run between pairs of nodes to form a ring-like path, (ii) at least one protection ring configured of second links that run generally parallel with the a corresponding first link; and (iii) a mesh sub-network network that includes a plurality of third links that provide point-to-point connectivity between pairs of the nodes. The service and protection rings are monitored to determine whether each link is in an active state (capable of carrying traffic) or a failed state (incapable of carrying traffic). The mesh network is monitored simultaneously monitored along with the service and protection rings links to determine the whether each third link is an active state or a failed state. Upon finding a failed link in one of the service rings and mesh network, a restoration path is designated, utilizing at least one link from a group of active second and third links, for carrying the traffic previously carried by the failed link. After establishing the restoration path, the traffic within the network is switched from the failed link onto the restoration path.